The primary non-invasive method of treating Obstructive Sleep Apnea (OSA) has traditionally used a Continuous Positive Airway Pressure (CPAP) device (also generically known as a Flow Generator), an interface to the patient's airway, and an interconnecting tube or pipe to supply a gas. There have also been further variations to this system including a flow generator that is integrated with a mask interface without an interconnecting tube (see U.S. Provisional Application No. 60/505,718 filed Sep. 25, 2003, and International Application No. PCT/AU04/01309, filed Sep. 27, 2004, each incorporated herein by reference in its entirety.
Traditional mask interfaces are configured to incorporate several fundamental features. They usually embody a frame, a sealing cushion, and headgear to secure the assembly to a patient's head, an inlet pipe for tubing attachment, and a vent that is usually part of a mask frame of elbow attached to the mask frame.
The mask vent for a CPAP mask allows a constant biased flow of gas from within the mask interface (gas exhalation region of a patient) to atmosphere (region external to a mask interface surrounding the patient). This constant or variable flow of air allows the partial pressure of waste exhaust gas from a patient's lungs (Carbon Dioxide or CO2), to be flushed to the atmosphere to prevent undesirable CO2 rebreathing or suffocation.
Mask vents have constantly proved to be a complex design issue for engineers who are required to develop vents that are quiet for patients and bed partners, flush out adequate CO2, and do not ‘jet’ irritating streams of air onto the body or bed partner due to poor dissipation or vented air direction.
The mask interface's vent flows outwards through one or a plurality of holes integrated within a mask interface that is in direct communication with the breathing gas chamber (inside of the mask frame). This occurs because there is a greater pressure inside the mask than the surrounding atmosphere. This is the basic principle of CPAP therapy, which raises the pressure within the patient's airways compared to the surrounding atmosphere.
The flow of fluid (air) can be explained by simple physics. A wall with a through-hole communicating with both sides (of the wall) with a higher pressure on one side will generally cause the air to flow towards the lower pressure side.
All existing CPAP masks have vented their patient-expired gases in the vicinity of the patient's head. The vent is either located on the mask frame or otherwise integrated into an attached component such as a rotating elbow, which is subsequently attached to the mask frame. See also U.S. Patent Publication No. 2001/0032648 A1.
This traditional vent location has provided an ideal location for CO2 washout but poor location for noise as it is relatively close to the patient's ears.
Carbon Dioxide Washout and Rebreathing
In general, greater flow through the vent holes will decrease CO2 rebreathing at a given pressure, as there is increased washout of expired gas. An undesirable effect typically results with increased vent flow, that is vent noise.
Adequate venting is generally a design issue at lower treatment pressures (say 4 cm water) where CO2 rebreathing is more likely. At higher pressures, the vent flow generally increases well beyond the safe limits required to prevent rebreathing, therefore the vent flow at high pressures may be mechanically and/or electrically reduced by variable vents and still maintain effective CO2 washout.
Vents have been designed to reduce the flow at higher treatment pressures, which may also provide reduced noise due to reduced vent flow. These variable vent valves, as some may describe in the art, are typically used to reduce the power required by a flow generator to deliver treatment pressures at the higher pressure ranges. One such variable vent example is known as the Respironics Plateau Valve. Other examples are ResMed's U.S. Non-Provisional application Ser. No. 10/433,980, filed Nov. 10, 2003, and ResMed's U.S. Provisional Application No. 60/640,184 filed Dec. 30, 2004, each incorporated herein by reference in its entirety. These vents, though lower in vent flow, are still an irritating noise source near the patient's (and bed partner's) ears.
Invasive ventilation is generally applied through the trachea and is often used where a patient is seriously ill and cannot effectively breathe on their own.
In Non-Invasive Positive Pressure Ventilation (also known as NiPPV or NPPV), a bi-level positive pressure device can be connected to the patient via a non-vented mask interface. These ventilators allow various modes of mechanical ventilation ranging from assisted breathing to fully controlled ventilation. In some cases, the machine can be set so that a patient can breathe almost naturally, receiving occasional air pressure to assist with individual breaths and has been known as assisted ventilation.
In sicker patients, the degree of ventilator driven respiration can be increased, and if necessary, the ventilator can take over the work of breathing entirely and has been known as Controlled Ventilation.
The expired air usually results from the patient's lungs deflating due to the wall elasticity after having been actively inflated by the machine. The gas outlet tube that is attached at one end to the mask interface is usually open at the other to atmosphere and works in similar fashion to a typical mask vent, where the machine incoming positive gas pressure ‘pushes’ the exhaust gas out from the patient interface. For example, the pressure within the mask pushes the exhausted air down a secondary tube that is controllable via a valving arrangement to detect exhalation of the patient and then vent the exhaled gas to either the ventilator or atmosphere.
Ventilators (as described above) differ from CPAP therapy. In CPAP, only sufficient gas flow to achieve adequate washout of CO2 needs to be vented to atmosphere. The patient does most of the work of breathing, not the device. The CO2 only represents a small partial pressure of the overall volume of air that is expired from a patient, therefore only a relatively small flow of air (for example 40-60 liters of intentional leak per minute at 20 cm of delivered pressure in the mask) is required to wash out adequate levels of CO2.
ResMed engineers have also shown that the gas flow dynamics inside a mask system have a significant effect on CO2 rebreathing, not only physical dead space as was previously understood. Moreover, the vent hole location and also designing a defined or biased flow of air (see ResMed's U.S. patent application Ser. No. 10/655,621, filed Sep. 5, 2003, incorporated by reference in its entirety, and commercially embodied in ResMed's Activa™ mask) reduces CO2 further. Designing a mask system to meet this requirement represents a significant design challenge with many tradeoffs such as noise and vented direction (and air jetting). The complexity of the mask design may also affect maintenance such as ease of cleaning and may negatively influence the mask configuration and appearance.
Mask interface internal dead space (the physical internal volume of the mask adjacent to but external to the patient's airways) also affects the CO2 rebreathing. Generally, increased volume results in increased rebreathing. The vent design, the location relative to the mask and patient airways, and impedance through the holes all affect rebreathing performance. A significant development effort is therefore required before a mask interface can be marketed and safely used on a patient.
Noise
Sleep apnea is a medical disorder that has been successfully treated globally using the non-invasive method known as CPAP. As the device is expected to operate in a home or hospital environment where patients and their bed partners are expected to sleep, it is a fundamental need for the treatment system to be quiet during use to minimize disturbance, irritation and discomfort.
Noise generally occurs as a flow of air through the vent(s) becomes turbulent and/or interferes with an object before dissipating to the atmosphere. Two main methods are employed by most manufacturers to reduce noise. Namely, producing vents that flow in a laminar state (smooth flow), and those that dissipate the vented gas. Laminar state vents generally attempt to smooth the flow of air as it exits a mask interface. In dissipation, the vented gas is passed through many tiny air passages to absorb sound energy.
Noise of vented air can be reduced using a multitude of methods. For example, many small holes can reduce noise as claimed in ResMed's U.S. Pat. No. 6,581,594, incorporated by reference in its entirety.
Noise may also be reduced using soft-edge holes design as disclosed in ResMed's “Soft-edge vent” patents (U.S. Pat. Nos. 6,561,190 and 6,561,191) and commercially applied in the Mirage® nasal mask and Mirage® Full-face masks.
Other methods utilize diffuse materials such as filter like filter wool over the vent holes such as that used on Fisher & Paykel's Aclaim™ mask.
Vents may also come as separate components that are subsequently attached at the mask inlet air entry point (e.g. elbow). These vents are considered attached to the mask assembly, though technically situated adjacent to a mask frame.
Air ‘slot’ vents have also been developed to reduce noise by causing the layer of vented air to pass through a relatively thin slotted-hole in order to reduce noise. Variations of this design include slots circumscribing an axis to the inlet pipe (e.g. Respironics Whisper Swivel vent) or a novel variation by ResMed that passes the air over a relatively long thin slot (e.g., see U.S. Pat. No. 6,691,707, incorporated by reference in its entirety, and commercially embodied in the ResMed Ultra Mirage® Nasal Mask).
All current CPAP mask interfaces vent at, near or adjacent to the mask interface. This results in a common irritating noise source in close proximity to a user (say within the diameter of a human head) and within a bed-width distance of a bed partner.
Air “Jetting”
Existing vent holes are fairly simple in nature. They are basically holes ‘drilled’ into mask components. These holes generate a thin jet of air that flows in the direction of the axis of the hole. This direction is considered by engineers to minimize disturbance of the patient and also the bed partner. Blowing vented air directly onto a bed partner (when facing each other) will be irritating and potentially affect treatment of the patient as one bed occupant's disturbed sleeping and movement may result in disturbance to the other.
The vent holes may also be angled to reduce direct interference, however, there is still the possibility of vented air blowing onto the patient especially when it is deflected by bedding materials.
The vent holes may also be mounted to rotating elbows that are attached to the mask frame. These rotating elbows traditionally provide freedom of tubing direction for the patient, e.g., over-the-head or downwards. Angled vents mounted to rotating elbows allow the vented direction to be selectable in the direction of the air tubing that is attached to the elbow. Although this is regarded as one of the better vent directions, it may still jet onto a patient or bed partner and/or create noise if interfered by bedding materials.
Another method of reducing the ‘jetting’ effect is by creating a more diffuse vent flow pattern. This may be achieved by the use of very small holes or by creating a multiplicity of tortuous paths for the vented gas. These vents may be difficult to clean, have minimal life, and can block easily with grit or even condensation (water droplets).
Whilst conflicting requirements will generally lead to a suitable balance acceptable to engineers, many consumers have their own preferences or requirements. For example, a patient may be satisfied with the vent noise level, however may be irritated by the chosen vent direction of the air jets.
Masks from DeVilbiss have also shown an air vent with selectable jet direction that may be rotated (user selectable) to a desired angle relative to the mask frame and therefore patient. However, sleep is an active exercise and the body will naturally move during the night or treatment session. A patient for example may not jet vented air onto their bed partner until part way through the sleep session when the patient's body or head moves, causing subsequent irritation and loss of quality sleep.
Pressure Swings and Breathing Comfort
Pressure swings is known in the art as the actual pressure change experienced at or near the patient airways between inhalation and exhalation. For example, a patient breathing on a flow generator machine set on 10 cm water pressure through a mask interface will experience a higher pressure on exhalation (say 12 cm), and a pressure drop on inhalation (say 8 cm); the resultant pressure swing in this example is 4 cm. Generally, the lower the swing results in less work of breathing on a patient, and therefore increased breathing comfort.
Excessive pressure swings can lead to non-compliance of a patient. Several products are available to treat these patients. One such device is known as a bi-level flow generator. Examples are the Respironics BiPap™ and the ResMed VPAP™ and can be set to reduce pressure on exhalation, therefore reducing the work of breathing (i.e. work of exhalation). Respironics also markets a C-Flex™ system where the device has improved breathing comfort by reducing the work of breathing by controlling the blower characteristics electronically.
Although bi-level flow generators are designed to produce pressure swings to treat the patient, breathing effort on exhalation can be modified so that minimal exhalation effort is required (that is more comfortable) when these devices are used on patients with sleep apnea who have unsuccessfully used a constant pressure CPAP device, as CPAP devices do not significantly reduce the effort required to exhale.
Other methods to reduce swings may include valves and also flow generators designs that have carefully considered blower design characteristics.
System Compatibility and Performance
Most manufacturers of CPAP systems develop mask interfaces and flow generators as a system where the flow generators are engineered to function optimally with that manufacturer's mask interfaces. Many CPAP machines do not however function optimally or even poorly with another manufacturers' masks, resulting in loss of performance and even comfort can be compromised.
Accordingly, a need has developed in the art for a system that addresses one or more or all of the above challenges.